This paper examines the techniques used and the results obtained in monitoring the water advance in a peripheral waterflood of a carbonate reservoir. The peripheral pattern used in the subject reservoir gives a water advance similar to that obtained in a water drive reservoir. However, monitoring this particular reservoir is complicated by the use of a low salinity brine for flooding and the areal shape of the reservoir.

The use of pulsed neutron capture logging in conjunction with production logging has been effective in differentiating between oil and water in porous zones in existing producers. The use of the two logs has been successful despite the problems normally encountered when logging open hole completions in a reservoir being flooded with a low salinity brine. Results have been confirmed and enhanced by open hole logs of new wells being drilled in the water invaded areas.

The paper highlights difficulties encountered, such as acid effect on the pulsed neutron capture log response and the effect of lower salinity flood water as the reservoir becomes better swept. The techniques used to overcome these problems is discussed. A number of examples are shown and a remaining oil map prepared from these surveillance techniques is examined.

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